Need some guidance about IR on a stock 350D

tsk1979

Hi,
I got myself an IR filter.
Since my 350D is stock, and my only cam, I am not removing the filter in front of sensor.
That means 3-4 seconds exposure in sunlight to get an image.
However there is a small problem
I am using 58mm filter, so it goes on my kit lens 18-55mm.
This leads to an ugly hotspot.
Is there a way in PP to remove hotspot.
Any aperture I can use which minimizes hotspot.
I currently shoot RAW and set WB by clicking on grass in Canon DPP.
This converts my purple-red image to greyish, as grass becomes grey-white


I was unable to find any generic tutorial(which can be used on GIMP) to remove hotspot.

Scott_Quier

I did some testing of my lenses on my converted 20D and found that none of them performed well (with respect to the light bloom in the center of the photo) above f/4.

I've not tested my 18-55 to see how it performs as I have other lenses in that range the I like a lot better.

As for correcting - Do you have CS3 or CS4? If so, there's a feature in ACR that can be used to correct lens vignetting. I think you can reverse this to darken (improve contrast) the center rather then lighten the corners. That might be worth the effort.

Another thought might be using a Curves adjustment layer to improve overall contrast in the photo and then apply a layer mask to "concentrate" that effect in the center of the image.

tsk1979

I use GIMP... No Photoshop

Scott_Quier

tsk1979 wrote:

I use GIMP... No Photoshop

See the last statement of my post

Scott_Quier wrote:

Another thought might be using a Curves adjustment layer to improve overall contrast in the photo and then apply a layer mask to "concentrate" that effect in the center of the image.

pathfinder

tsk1979 wrote:

Hi,
I got myself an IR filter.
Since my 350D is stock, and my only cam, I am not removing the filter in front of sensor.
That means 3-4 seconds exposure in sunlight to get an image.
However there is a small problem
I am using 58mm filter, so it goes on my kit lens 18-55mm.
This leads to an ugly hotspot.
Is there a way in PP to remove hotspot.
Any aperture I can use which minimizes hotspot.
I currently shoot RAW and set WB by clicking on grass in Canon DPP.
This converts my purple-red image to greyish, as grass becomes grey-white


I was unable to find any generic tutorial(which can be used on GIMP) to remove hotspot.

Some lenses are more prone to hot spots than others, and generally, zooms are not the best choice for IR
Here is a link to a list of lenses than are known to work poorly for IR shooting - http://heim.ifi.uio.no/%7egisle/photo/ir.html#lenses

As you can see, the 18-55 is known to give hot spots.

I found out the hard way that the well liked 85mm f1.8 is also a poor choice for IR due to hot spots.

I think it is easier to use a better lens for IR, than trying to salvage an image with a real hot spot, but that is just me.

I find the 17-40 f4 L works very nice for IR on a 300D.

ziggy53

pathfinder wrote:

...

I find the 17-40 f4 L works very nice for IR on a 300D.

I can confirm that the EF 17-40mm, f4L USM works on an IR converted XT/350D as well.

In order to help with a hotspot you can produce an image by photographing a white card at close distance (it does not have to be in focus but it needs to cover the entire field of view) with the offending lens at the same focal length as you wish to use the lens. This will produce an image with pretty much "just" the hotspot.

Use a competent image editor to invert the image colors and it can be used as a color correction mask for using against offending images. You may have to blend the 2 layers and adjust the colors of the mask somewhat but I use this technique on a Minolta DiMAGE 7 digicam with an IR filter and it works pretty well to even out the hotspot. (Actually I wound up producing a PhotoShopped image using the actual inverted "hotspot only" image as a guide to the size and colors needed.) I suppose that using a white diffuser in front of the lens/filter would produce the best image to start with for the mask.

tsk1979

Thanks!
I will try the grey/white card trip, get a hotspot image.
MEanwhile I am attaching a sample shot I took.
I focussed without filter(with filter it can't focus) put on filter and took a 3.2" exposure in bright sunlight
Here is the result. All I did was custom white balance adjustment of RAW file.No desaturation and no color mixing


I am wondering how do people get those wonderful false colors? Even though I did not desaturate it, my pic is coming desaturated.
Is it because my filter is 870nm cutoff one and not R72 or something?

ziggy53

Here is the hotspot from the Minolta digicam:



... and here is the corrective mask I built in PhotoShop for a typical tungston WB:



This would yield a fairly good starting point for the rest of the conversion. Note that your hotspot is much more pronounced and so you would probably need a more corrective mask. You might also need multiple exposures that would need to be blended in software because your hotspot is affecting the exposure so much.

BTW, the Canon 50mm, f1.8 is a fairly good choice for IR on a budget.

For the Canon XT/350D if you use AF and then add the IR filter you would also need to adjust focus as IR focuses closer than visible light images.

For an IR processing false-color tutorial try this tut:

http://stevencastle.wordpress.com/2008/07/17/digital-infrared-processing-part-ii-false-colour/

tsk1979

I decided to use the 50mm lens.
I shot IR(2 second exposure in bright sunlight)
Then I opened up Canon DPP, the image was magentish. I clicked on the white tree leaves for click white balance, and the image turned monochrome.
I went through the steps of Removing green channel to get false color, but no luck.
It seems my filter is not 720nm cutoff, but a 850nm cutoff, so I have true IR, without any color information.
This means I can't get false color?
Anyways here are the samples.
1. This pics I took through the false color tutorial. I get a hint of color in the sky, thats all

2. In this pic there is not even a hit of Color! Even though I did not de-saturate it


Some pointers on how to get false color?

Scott_Quier

tsk1979 wrote:

It seems my filter is not 720nm cutoff, but a 850nm cutoff, so I have true IR, without any color information.This means I can't get false color?
Anyways here are the samples.

Take a look at this page, in the bottom right square of the embedded table. The cameras modified with the 830nm "...Notice how the 830nm color channels are more closely balanced than the 715nm." This "better balance" means that the RGB channels have approximately record the same luminosity values for a particular point in the scene. The same numbers mean that you will get and essentially monochrome image - there is no information recorded with which color to descriminate between one color and another.

tsk1979

Thanks scott!
Looks like what I lose in false color, I gain in IR effect.
But to be sure, I would like to mail some expert here my RAW file.
I think a PS expert can do PP and then let me know whether my filter is 720nm or 850nm.
Since its a chinese massa filter, there are no markings

Scott_Quier

tsk1979 wrote:

Thanks scott!
Looks like what I lose in false color, I gain in IR effect.
But to be sure, I would like to mail some expert here my RAW file.
I think a PS expert can do PP and then let me know whether my filter is 720nm or 850nm.
Since its a chinese massa filter, there are no markings

I would love to help you out, but I'm a very far cry from being an expert.

tsk1979

Thanks scott!
Can you PM me your email address? I will mail you the RAW file. Its 8.08 MB.
I think I will upload to rapidshare or something and send you the link

Scott_Quier

tsk1979 wrote:

Thanks scott!
Can you PM me your email address? I will mail you the RAW file. Its 8.08 MB.
I think I will upload to rapidshare or something and send you the link

Scott_Quier wrote:

I would love to help you out, but I'm a very far cry from being an expert.

This is/was my way of saying that I can't really help you with that.

I'm sorry - I wasn't clear. I would help you if I knew enough to be of help. But, I don't. I can process the RAW files I get (which are also 815nm - so no color), but I've never been able to do the false color thing. I even had the opportunity to use a friends 730nm IR camera for a day and wasn't able to get pleasing results.

I hope you can find someone to help you out.

tsk1979

ACtually I wanted to know how to find out whether the filter is 730nm or 815/830nm. My filter has no markings. So no way to tell.